Soybean sudden death syndrome (SDS) has always been a head-scratcher. The late-season disease has caused losses across the Soybean Belt since it was discovered in Arkansas in 1971.
Last year, the disease turned particularly ugly and Iowa State University plant pathologist X.B. Yang started getting reports that SDS wasn’t exhibiting textbook behavior.
"The 2010 outbreaks indicate many SDS management recommendations need more thought," Yang says. Growers typically recognize SDS symptoms late in the season when there is little recourse. Proactive moves include knowing the fields that had the disease and taking proper steps at planting.
Yang says the rumor mill likes to link glyphosate applications and SDS. "This came up more than a decade ago when Roundup Ready soybeans hit the market," he notes. "Our plot data shows labeled rates of glyphosate had similar levels of SDS as plots that received Pursuit, the popular herbicide prior to the Roundup Ready system."
However, Yang notes, plots receiving twice the labeled glyphosate rate did exhibit an increased incidence of SDS. He suggests that growers who upped herbicide rates to tackle tough weed problems consider this as they work to manage SDS.
Control measures. "In 2010, high SDS levels were found in Roundup Ready and non–Roundup Ready soybean fields," Yang says. Some believe Cobra (lactofen) can reduce SDS, but studies from 1997 to 1999 found that to be true only in the greenhouse, he adds.
Field trials during those same three years did not show Cobra to be effective, Yang says. He revisited the Cobra question in 2008 with multi-location trials in eastern Iowa with the same results. In 2010, he says, postemergence Cobra applications caused a yield penalty due to a lack of white mold.
Seed treatments haven’t shown any benefit in battling SDS either. While new chemicals are in the pipeline, good field results are needed before they can be recommended, Yang says.
Drainage is typically at the top of many SDS management recommendations. However, floods in 2010 put a crimp in this theory. "Many well-tiled fields had severe SDS and the disease was often more severe in line with drainage pipes," Yang says. When it comes to SDS, tiling seems to help more in years of normal weather conditions, he adds.
One link is certain: The presence of soybean cyst nematode (SCN) can enhance the severity of SDS. "What happened in 2009 and 2010 shows that SDS can strike without SCN," Yang says. He observes that SCN was suppressed in many parts of the Midwest because the summer of 2009 was one of the coolest in history and 2010 was one of the wettest. In both years, though, SDS was prevalent despite the low SCN levels.
"This suggests that SCN management may help reduce SDS in years that have normal weather conditions, not years like 2009 or 2010," he says.
SDS is also associated with early planting. University of Illinois plant pathologist Carl Bradley recommends planting fields with no history of SDS problems first. "Soybean fields planted in late April may be affected most by SDS, especially when followed by cool, wet weather," he says.
Variety selection remains your best defense—unless you’re sure the coming season is likely to be dry.
"Susceptible varieties tend to yield a few bushels more when they are disease-free," Bradley says. "One year of SDS can cut profits enough that it can take several years to catch up."
Resistance is not as reliable as it is for other diseases, such as phytoph-thora. Soybean breeders have made progress, but SDS’s unpredictable nature makes it a tough opponent.